The invention relates to the use of pressure retarded osmosis (PRO) to provide direct current electricity (DC) to continuously charge the battery of an electrically-powered vehicle (EV), or other mobile electric conveyances such as a railroad car, a battery powered electric aircraft, or a submarine. Thus a PRO-Electric Hybrid is an autonomously powered electric vehicle or machine that would otherwise need to be plugged into an electric charger powered by the electrical grid. The term hybrid as used in this document refers to the combined use of two separate technologies, a power production device (a PRO Unit) and either a battery or a capacitor device. More particularly, it relates to a PRO unit that exploits the combined use of osmotic pressure, a water-submerged hollow fiber membrane system, a concentrated aqueous solution of superparamagnetic nanoparticles (a ferrofluid) as a draw solute, and a solenoid-type permanent magnetic field to maintain the position of the ferrofluid within the hollow fiber membrane strands, to create a high pressure water flow that acts upon one or more hydroturbine generators to produce electricity. After the pressurized water acts upon the hydroturbine, it is returned to the feed side of the membrane system to once again become permeate, in effect making the entire system a closed loop, continuously re-circulating process.
Osmosis is the natural, universal and biologically ubiquitous movement of a solvent, such as water, through a selectively semi-permeable membrane from a region of low solute concentration solution to a region of comparably high solute concentration solution. The selectivity of the membrane allows for the passage of the solvent while preventing the passage of larger solute molecules through the relatively smaller membrane pores. The natural passage of water through a membrane is driven by the difference in the solute concentrations on either side of the membrane. The larger the difference between the solute concentrations, the greater the driving force of the solvent to permeate the membrane. In other words and in accordance with the second law of thermodynamics, each of the solutions separated by the semi-permeable membrane seeks an equivalent solute concentration. If the membrane contains pores of a size that prevents the passage of solute molecules or particles, (solute particles refer to superparamagnetic nanoparticles in this invention), and only allow for passage of the solvent, then the solvent (the pure water in this invention) will permeate across the membrane according to the osmotic pressure that is proportional to the two different solution concentrations. If two differing concentration solutions are separated by a semi-permeable membrane, the solvent of the lower concentration solution will pass the membrane to dilute the higher concentration solution, until the concentrations of the two solutions are equal. This driving force is known as osmotic pressure. When resistive forces are applied to this osmotic force, the result is referred to the net osmotic driving force. Since this resistive force retards the original osmotic pressure, it is called pressure retarded osmosis (PRO). This resistive force can be the performance of work, such as action upon a hydroturbine generator to create electricity.
The relationship between the concentration of superparamagnetic nanoparticles on the draw solution side of the membrane, the concentration of the solute on the feed side of the membrane, and the osmotic pressure of permeate water, is of key importance. The higher the difference between the solution concentrations, the higher the osmotic pressure, and the higher the resulting permeate flux rate. The higher the permeate flux rate acting upon the hydroturbine, the higher the electrical generation potential.
The general equation describing water flux permeating a membrane in a PRO system is known as the Morse Equation. Jacobus Henricus van't Hoff (30 Aug. 1852-1 Mar. 1911) first proposed a formula for calculating osmotic pressure for which he was awarded the inaugural Nobel Prize in Chemistry in 1901. This formula was later improved upon by Harmon Northrop Morse (Oct. 15, 1848-Sep. 8, 1920). The current formula is known as the Morse Equation, and is defined as:JW=A(σΔπ−ΔP), where                JW is the water flux,        A is the water permeability constant of the membrane        σ is the reflection coefficient        Δπ is the osmotic pressure differential        ΔP is the applied pressure.        
The osmotic pressure, π=iMRT, where                i is the dissociation factor or the dimensionless van't Hoff factor        M is the molarity or the concentration of the solution, moles/L        R is the universal gas constant        T is the absolute temperature, degrees Kelvin        
The Morse equation can be used to calculate each solution's flux potential on each side of the membrane. The difference between the two pressures on either side of the membrane (Δπ), becomes the total pressure of the membrane system. Therefore, the total pressure of the PRO system, Δπ, is equal to (iMRT)f−(iMRT)p, where f is the feedwater side of the membrane, and p is the permeate side of the membrane. Assuming that each side of the membrane is at the same temperature, and R is constant, [(RT)f=(RT)p], then the Morse equation mathematically simplifies to the linear relationship, Δπ=(iM)f−(iM)p. In other words, the total pressure of the membrane system is proportional to the difference of the concentrations of each of the solutions, times its solute's van't Hoff factor.
In practice, the actual measured osmotic pressure across the membrane is much lower than is described by the Morse Equation, resulting in much lower water flux. The lower than expected water flux rate is attributed to several membrane-related transport phenomena. In particular, two types of concentration polarization (CP) phenomena, concentrative CP and dilutive CP, can take place. Furthermore, concentrative and dilutive CP can each take place both internal and external to the membrane.
Concentrative CP occurs in osmotic systems in which the feedwater is a solution of pure water with a dissolved solute, and the membrane effectively rejects the feedwater solute molecules and ions. As pure water from the feed solution permeates the membrane, there is a buildup of solute material left in the feed solution at the active layer surface. As a result, the feedwater solution becomes more concentrated within close proximity of active layer of the membrane. The increased solute concentration of the feedwater solution, increases its osmotic pressure, and decreases the net osmotic driving force of the membrane system. If solute ions bleed into the either the dense separating layer or the porous support layer of the membrane, the Concentration Polarization is referred to as internal concentrative CP.
If the concentrative CP is extreme, the dissolved ions can precipitate and form a particulate scale on and within the membrane, further reducing mass transfer.
Likewise, on the draw side of the membrane, the pure water permeating the membrane dilutes the draw solution, further reducing the net osmotic pressure of the membrane system. CP is considered one of the most important measures to be taken in order to advance the field of forward osmosis and pressure retarded osmosis.
This invention addresses both concentrative and dilutive CP in a revolutionary manner. First, the feedwater is pure water, and therefore no concentration phenomenon can occur. Second, since the osmotic draw solute consists of superparamagnetic nanoparticles, and their position can be controlled by a permanent magnet, they can be effectively pushed to within close proximity of the membrane. Therefore, the strength of the solenoid-type magnetic field can be selected in order that the nanoparticle can overcome the flow of the incoming permeate. This way, the particles can be effectively positioned near the membrane and be maintained as effective draw solutes.
Third, the membrane can have relatively large pores because its purpose is not to prevent the passage of dissolved ions, it is to prevent the passage of much larger draw solute particles. The pores need only be small enough to prevent the passage of the draw solutes across the membrane. As a result, the membrane with its large pores will have a high permeability constant which will allow a high water flux. The high water flux will enable high electricity generation rates.
For most non-electrolytes dissolved in water, the van' t Hoff factor1 is essentially one. For most ionic compounds dissolved in water, the van't Hoff factor is equal to the number of discrete ions in a formula unit of the substance. Since nanoparticles are not ‘dissolved’ in water, their van't Hoff factor needs to be accounted for in some other way. Also, nanoparticles need to be chemically treated with a dispersant, a so called surface ligand, to keep them suspended in a stable aqueous suspension. Otherwise, the particle would settle in the water solution, or adhere to and agglomerate with other particles. Researchers Ge2, et al have approximated the molecular weight of the surface ligands surrounding each magnetic nanoparticle to quantify the osmotic pressure of the draw solution They report that this is appropriate since it is the surface ligands that function to extract, or pull the water across the membrane in the forward osmosis process.
Despite the fact that the Morse equation, the Ge researchers, and nearly all biological texts call the “high” osmotic pressure side of the membrane, the side with high concentration of solute particles, the choice made in this document is the opposite. The rationale for this choice is that the energy which drives the fluid transfer is the thermal, vibrational energy of the water molecules, not the thermal energy of the solute particles, because the solute particles do not contribute to fluid transfer across the membrane due to their size relative to the membrane's pore size. Other source documents describe osmotic pressure, and specifically which side of the membrane to call the “high” osmotic pressure side, as is done here. Researchers at other academic institutions, e.g., Georgia State University3, also attribute the high pressure side to the pure-water, solvent side of the membrane. In other words, the pure water side of the membrane contains higher energy density because of the higher relative concentration of water molecules. The vibrational energy of the nanoparticles does not contribute to osmotic pressure because they cannot permeate the membrane. The no-energy draw solute nanoparticles volumetrically displace the high-energy water molecules on the draw solution side of the membrane. Van't Hoff factors greater than one as attributed to solutions containing multi-ion formula compounds, have a volumetric explanation for their greater osmotic pressure generation: the dissolved multi-ionic compound takes up more volume, displaces more water, and therefore results in greater osmotic pressure generation. Furthermore, the Morse equation's rationalization of why a solution of multi-ion compounds generates a higher pressure can be explained by the increased volumetric displacement of pure water by the multi-ionic compounds. Multi-ion compounds displace more water than single-ion compounds.
Resistive forces can be applied to this osmotic force. Since the resistive force retards the original osmotic pressure, the phenomenon is called pressure retarded osmosis (PRO). The resistive force can be the performance of mechanical work, such as action upon a hydroturbine generator to create electricity. The resistive force provided by the hydroturbine can be made variable and adaptable so that maximum energy is transferred from the pressurized water stream to the turbine (even with potentially varying osmotic flux performance and resulting changing water pressure) while allowing enough remaining pressure for the water stream to return to the feed side of the pure-water bath. This can be accomplished by configuring the PRO system with multiple (two or more) hydroturbines, of reducing pressure stages, in series, starting with the highest pressure stage and ending with the lowest pressure hydroturbine stage.
The solute particles used in this PRO unit are superparamagnetic nanoparticles. The unique characteristic of a superparamagnetic nanoparticle is that it only exhibits magnetic behavior while it is in a magnetic field. In the absence of an externally applied magnetic field the nanoparticles exhibit neither remanence (the measure of particle's remaining magnetization when magnetic field is zero), nor coercivity (the measure of the reverse field needed for the particle to become demagnetized). When these particles are in a liquid water solution, the solution is called either an aqueous or a water-based ferrofluid.
Since the physical location of the ferrofluid can be controlled by a magnetic field, a ferrofluid is an ideal PRO draw solution. The ferrofluid's presence at the surface of the membrane can be maintained, and thus the particle can be prevented from being swept away from the membrane by the incoming permeate flow. It obviates the need to separate the nanoparticles from the membrane effluent and avoids the need to continuously replace the relative costly ferrofluid. Furthermore, maintaining the ferrofluid inside the membrane with a slight force, reduces the effect of dilutive, concentration polarization, a phenomenon that reduces osmotic pressure and correspondingly, the flux rate and electricity generation potential of the PRO Unit.
The magnetic field shape used in this invention is a solenoid magnetic field. It can either be created with permanent bar magnets placed in a cylinder configuration or with hollow electromagnetic coil. Both permanent solenoid magnets and electromagnetic solenoid magnets are used to repel the superparamagnetic nanoparticles in this PRO invention.
The usual application of pressure retarded osmosis is to desalinate seawater and at the same time produce electricity as taught by Loeb in U.S. Pat. No. 4,193,267, by Al-Jlil in U.S. Pat. No. 8,197,693, and in a hybrid RO/PRO system by Stover, et. al. in U.S. Pat. No. 7,871,522. The key basic characteristics of an osmosis system, in addition to the type of osmosis, are the type and composition of the membrane; and the type and composition of the draw solution. Membranes can either be flat sheet membranes in a plate-and-frame configuration or in a spiral-wound configuration, or could be tubular. In turn, tubular membranes can be either tubes or hollow fibers. Cath et al4 discuss the advantages of hollow fiber membranes. They point out that hollow fiber membranes can support high hydraulic pressure without deforming and can be easily packed in bundles directly within a holding vessel. They are also relatively easy to fabricate in modular form. Also, they allow liquids to flow freely on the feed side of the membrane. Other advantages of hollow fiber membranes are they are much cheaper to manufacture and they can have several hundred times the surface area per unit volume than flat sheet or spiral wound membranes. In U.S. patent application Ser. No. 13/987,129, Aylesworth teaches the use of a PRO system using hollow fiber membranes for producing electricity for more general stationary applications than concurrent desalination.
A variety of compositions can be used for the draw solution. In an early commercial application of forward osmosis (FO), Wickenden in U.S. Pat. No. 2,116,920 teaches the use of calcium chloride as a draw solution in the concentration of fruit juices. In another early patent, Batchelder in U.S. Pat. No. 3,171,799 teaches the use of a volatile solute, such as sulfur dioxide, in a draw solution for the demineralization of water. Recently interest in draw solutions has centered on those containing magnetic nanoparticles. Magnetic particles in the draw solution have the advantage of being readily separated from the product water of a purification or desalination process with use of magnetic fields. They can also be readily recycled back into the draw solution. A kind of nanoparticles that is currently of interest is a material referred to as magnetoferritins. As Oriard et al describe in US 2007/0278153, it is magnetite bound to a protein such as ferritin wherein the magnetite is the core and the protein is the spherical cover. The use of magnetite nanoparticles is also taught by Etemad et al in US 2010/0051557 in the context of removing heavy metals from aqueous media by means of adsorption and magnetic capturing. Etemad et al mention that the magnetite is superparamagnetic but does not indicate whether they are coated with a protein. Superparamagnetic iron oxide nanoparticles (SPIONs) are also the subject of intense research for various biomedical applications as described by Latorre et al5.
An increase in feedwater temperature in an osmotically driven process can increase the flux considerably. Kim and Elimelech6 report that an increase in feedwater temperature from 20° to 30° C., (from 68° to 86° F.) will result in an increase in water flux of 50.5%, from 9.23 to 13.89 L/m2h, for a resulting power density increase of 46.6%, from 3.22 to 4.72 W/m2 